Single magnet tandem mass spectrometer

ABSTRACT

A mass analyzer utilizing a single magnet for providing two mass analyzing sectors in tandem. The analyzer is equipped with a magnet defining an ion trajectory subtending an angle of deflection between 180* and 360*. A slit is located within the magnet at a first point of focus for substantially limiting ions passing beyond the slit to those of a predetermined mass.

United States Patent 11 1 11] 3,783,278 Cambey Jan. 1, 1974 [54] SINGLEMAGNET TANDEM MASS 2,680,812 6/1954 Berry 250 41.9 SPECTROMETER3,492,480 1/1970 Vogel 250/419 X I 3,247,373 4/1966 Herzog et a1.250/419 Inventor: Leslie y, a ,C 2,932,738 4/1960 Bruck 250 4193,126,477 3/1964 Noda et al.. 250/419 [73] Assgnee' & Company 3,379,9114/1968 Enge 250/419 x 3,500,042 3 1970 Castaing eta1.... 250 419 X [2Filed: 1971 FOREIGN PATENTS OR APP CATIONS [21] Appl. No.: 204,336141,559 12/1960 UAS.S.R 250/419 Application Primary ExaminerWilliam F.Lindquist [63] Continuation of Scr. No. 819,840, Apr11 28, 1969. Atl0meyEugene F Friedman 52 us. 01 ..250/299, 250/397 511 Int. Cl. 110m 39/34[57] -K 581 Field of Search 250/419 0, 41.9 D, A mass analyzer a 5magnet for Pmvldm? 250/419 ME two mass analyzing sectors n tandem. Theanalyzer 1s equipped with a magnet defining an ion trajectory sub [56]References Cited tending an angle of deflection between 180 and 360.UNITED STATES PATENTS A slit is located within the magnet at a firstpoint of focus for substantially limiting ions passing beyond the g zslit to those of a predetermined mass. y e a 3,231,735 1/1966 Peters250/419 17 Claims, 6 Drawing Figures 1. SINGLE MAGNET TANDEM MASSSPECTROMETER This is a continuation of application Ser. No. 819,840,filed Apr. 28, 1969.

BACKGROUND OF THE INVENTION This invention relates to mass analyzers andin particular to analyzers providing tandem magnetic mass analyzingsectors between a source of particles to be analyzed and a collector forthe charged particles.

Tandem mass spectrometers'utilizing two separate analyzers have beenpreviously known. See, for example, U.S. Pat. No. 3,231,735 and a papertitled A Two Stage Magnetic Analyzer for Isotopic Ratio Determinationsof to l or Greater, White and Collins, ASTM Committee, E-l4 on MassSpectrometry, New Orleans, Louisiana, May 1954. As is indicated in thereferenced patent and article, such tandem mass spectrometers are usedin leak detection applications and in isotope ratio or abundancedeterminations. Mass spectrometers of this type are useful in suchapplications because of their inherently better ability to detect thepresence of a mass or certain masses of predetermined values when onlysmall traces of such masses are available or when the pressure of thesample being analyzed is at a relatively high value. Such analyzers areparticularly effective in reducing the effect of gas scattering and ionsformed by intermolecular processes.

As indicated, the present invention provides an in-,

strument which falls under the broad heading of tandem mass analyzer,but is an improvement over the conventional tandem instrument in thatthe two succesive analyses which are performed on a sample of ions inthe course of their passage from an ion source to an ion collector areaccomplished with a single magnet. As set forth in the '735 patentabove, the passage of ions through a single stage ofdirectionalre solutiofi allows a substantial quantity of unwanted ions to pass throughthe analyzer and arrive at the collector producing broad and poorlydefined mass peaks at the analyzer recorder. Under such circumstances itis difficult and sometimes impossible to determinewhether ions of apredetermined mass are present in the sample, particularly when thesample includes 'only trace quantities of such ions. However, bysubjecting the ions in the sampie to successive stages of singlefocusing resolution, elimination of a substantial portion of theunwanted ions is possible giving more nearly ideal peaks on the analyzerrecording apparatus. Thus, a tandem instrument in comparison to a singleanalyzing sector instrument has increased sensitivity, is greatlyimproved in its capability of reducing spurious response and also morereadily lends itself to use with secondary emission collectors.

SUMMARY OF THE INVENTION with respect to the exit boundary so as toreceive particles from the sector emerging at a second predeterminedangle to a normal to the exit boundary, the collector being spaced apredetermined distance from the exit boundary. The source, collector andanalyzing sector define a charged particle trajectory having a firstpoint of focus within the anaylzing sector for particles of apredetermined mass and a second point of focus for said particles at apoint intermediate the exit boundary and the collector. Finally anaperture is located within the analyzing sector coinciding with thefirst point of focus.

The invention also contemplates a mass analyzing apparatus comprising amagnet defining a first and second analyzing magnetic field in tandemwith the magnet having an entrance and an exit end. A source of chargedparticles is arranged to direct said particles into the entrance of themagnet and means for detecting the charged particles emerging from theexit of the magnet is also provided.

The invention further contemplates a magnetic analyzer for use in massresolving systems comprising a pair of spatially separated magnetic polemeans for creating an arcuate analyzing magnetic field therebetweenhaving an entrance and an exit end. The field is arranged so as to havean arc in excess of whereby a first point of focus for charged particlesof a predetermined mass is created interiorly of the magnetic fieldintermediate the entrance and exit ends of the field and a second pointof focus for charged particles of said mass is created on the side ofthe exit end of the field opposite the first focus point.

A principal advantage of the present invention is that of economy ofmanufacture in that a tandem mass spectrometer can now be provided.which requires only a single magnet for creating the two mass analyzingfields. In a particular embodiment of the present invention a furtheradvantage, that of stigmatic focusing, is also obtained, furtherenhancing the sensitivity of the instrument and preventing the loss ordefocusing of ions in their passage along the analyzer axis. As with thepriorart tandem mass spectrometers, it is contemplated that the presentinvention will have its primary application in leak detection andisotopic abundance work. Further refinements of a mass analyzeraccording to the present invention are also possible as will be shown inthe following detailed description of the in vention.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be betterunderstood by reference to the following figures in which:

FIG. 1 is a plan view in section of a tandem mass spectrometer accordingto the present invention;

FIG. 2 is a simplified sectional view taken along lines 22 of FIG. 1;

FIG. 3 is a circuit diagram illustrating the various electrical controlsof the apparatus of FIG. 1;

FIGS. 4A and 4B are illustrations of alternate embodiments of thepresent invention; and

FIG. 5 is an enlarged sectional view of the exit end of the magneticanalyzing sector of the invention ineluding a pair of ion deflectingplates.

DESCRIPTION OF A SPECIFIC EMBODIMENT:

A mass analyzer 10 according to the present invention is shown in FIGS.1 and 2. The analyzer includes an ion source 12, an ion collector 14 andan analyzing sector 16. Also shown is a connection 18 from the interiorof the analyzer to a vacuum system (not shown) for lowering the pressurein the analyzer to the desired level. A feedthrough tube 20 provideselectrical connections to the various electrodes of the analyzer fromexteriorly located power sources.

The ion source 12 houses a source of electrons (see FIG. 3) and variouselectrodes associated with the ion source of a mass analyzer including asplit focusing electrode 22 located between the ion source and anelectrode 24 defining an object slit 25. Gas molecules to be ionized areintroduced into the ion source through a port 26. Once in the ion sourcethe molecules are ionized by electron bombardment and by suitablerepelling potentials on the electrodes within the source are urgedtoward the focusing electrode 22 and the object slit 25.

The electrodes of the source, particularly focusing electrode 22, areelectrostatically energized such that the ions emerging from the housingare focused at the center of slit 25 to provide that ions directedtoward the analyzing sector emanate from a point source 28. Theboundaries of sector 16 are defined by pole pieces which create acharged particle angle of deflection of 270. The particles entering themagnetic field created by pole pieces 30 experience a force tending tocause them to travel in an arcuately shaped path 32 toward anintermediate slit 34 located midway between the point of entrance 36 tothe magnetic analyzing sector and the point of exit 38 from said sector.Due to the geometry of the magnetic analyzing sector as will bediscussed ,in more detail below, the charged particles (ions) enteringthe entrance end 36 of the analyzing sector are swept through the curvedpath with ions of a predetermined mass coming to a first point of focus40 located in the center of intermediate slit 34. Slit 34 (shownenlarged in FIG. 1 for purposes of illustration) is chosen of such awidth dimension that ions of other than the predetermined mass to chargeratio are retarded from passing therethrough. In this manner the numberof ions contributing to the general background and noise level arelimited to a relatively low figure prior to the subjecting of ions ofthe predetermined mass to a second stage of analysis or resolution inthe area of the magnetic field between the intermediate slit 34 and theexit end 38 from the magnetic analyzing sector. Ions emerging from slit34 travel in an arcuately shaped path 42 and converge toward a point offocus 44 located in the center of an image slit (not shown) mounted insupport 46 defining the entrance into the collector 14.

A retarding lens 48 is provided between the image point 44 and thecollector 14 to establish a potential barrier between these two pointswhich retards the passage of ions of undesired mass to charge ratio. Theoccurrence of such ions is due to a wide angle scattering effectexperienced by some ions in their passage through the analyzer causingthem to lose a certain amount of energy originally imparted thereto andleaving them with a momentum corresponding to the momentum of ions ofthe desired mass to charge ratio. Thus. witlmht the provision ofretarding lens 48 such ions would pass to the collector. Ily crcnting npotential barrier only ions having a. sullicicntly high momentum.namely. those having the desired mass to charge ration are admitted intothe collector portion of the analyzer.

An auxiliary collector 37 is positioned at a location interiorly removeda relatively small distance from the entrance to the magnetic sector 16.In leak detection applications, the primary tracer gas, e.g., heliumfollows the main ion trajectory 33, 42 to collector 14. An auxiliarytracer gas, e.g., hydrogen is collected by collector 37.

The configuration and operating parameters of the analyzer according tothe present invention depend upon the selection of the angle ofdeflection through which the ions to be anaylzed are to be passed.Starting with the premise that two analyzing sections are to beprovided, the total angle of deflection to be provided is in excess of180. If two symmetrical analyzing sections are employed, a total singledeflection to 270 has been found to be convenient. Thus, the angle ofdeflection for each of the two sections is Median focusing of the ionswithin the plane of FIG. 1 occurs at focus 44. Further, if Z-focusing isdesired, i.e., provision against loss of ions due to drifting in avertical direction above and below the optical axis of the analyzer, theobject slit should be placed at the focal point of the Z lens. Thismeans that the point of 2 focus is located at the same point that theimage slit is spaced from the exit end of the magnet boundary. Relatingthis condition to the radius of curvature of the ions within the magnet,it can be shown that the angle between the ion trajectory at the exit orimage side of the magnet and a normal to the pole boundary is 2635. Thisfixes the location of the object s litandfor tIiEradiHSBTc'urvature inthe analyzer of FIGS. 1 and 2 means that the ratio of the distance ofthe object slit from the entrance magnetic boundary to the radius ofcurvature is 1.998. For the analyzer shown in FIGS. 1 and 2 thepertinent optical information is:

where 1' is the distance between the object slit and the entrancemagnetic boundary, a,,, is the radius of curvature for ions of apredetermined mass in the magnetic field of the analyzing sector; e isthe angle between the ion trajectory and a normal to the magnetic poleboundary; 1 is the angle of deflection for particles of a predeterminedmass in each of the analyzing sectors of the analyzer; 1",, is thedistance between the image slit and the exit magnetic boundary; e" isthe angle between the ion trajectory and a normal to the exit poleboundary. Thus, for a given angle of deflection and a given requirementfor Z-focusing, the angle which the ion path makes to the pole boundaryand the spacing of the image and object slit from the pole boundariesfor a given radius of curvature is specified. At a magnet angle ofdeflection of the image and object slit are located at infinity. At anangle of 360 the image and object slit coincide. Therefore, due topractical considerations inherent in providing a useful instrument, ananalyzer providing an angle of deflection greater than 180 and less than360 is contemplated by the present invention. Alternate embodiments ofthe analyzer shown in FIG. I are shown in FIGS. 4A nnd 413 having nnglesof deflection of 240 and 300", respectively. Referring to these figuresbriefly, an object slit (not shown) is positioned at point source todirect ions into a magnetic analyzing sector 52 having an intermediatepoint of focus 54 and an image slit (not shown) located at a secondpoint of focus 56 beyond the exit from analyzing sector 52. In FIG. 4B,the embodiment of the analyzer having an angle of deflection of 300, apoint source of ions is lo cated at 58 and an image point is located at60 with the ions being resolved by magnet 62 to a first intermediatepoint of focus 64 and thereafter to the second point of focus 60, thepoint of location of the image slit.

The electrical controls of a mass analyzer according to the presentinvention are shown in FIG. 3. A power supply 66 is connected to apotentiometer 68 having a plurality of taps connected to various partsof the analyzer. Tap 70 is connected to a repeller plate 72 located onthe side of an electron generating filament 74 opposite the object slit24. A second tap 76 is connected to a boundary electrode 78 which,together with repeller plate 72, define an ion chamber surroundingfilament 74. Tap 80 is connected to an emission regulator 82 forregulating the production of electrons from filament 74 which bombardsample molecules admitted to the ion source. Tap 84 is connected toelectron focus electrode 86 and provides means for controlling the focusof the electrons generated by filament 74 to maximize the ionizationefficiency of the ion source. Taps 88 and 90 are connected to split ionfocusing electrodes 92 and 94, respectively, for providing precisecontrol of the ion focus to locate the point source ofions in the centerof object slit 24.

Tap 96 is connected to retarding plate 48 for providing the potentialbarrier referred to in connection with FIGS. 1 and 2. Ions emerging fromthe analyzing section of the instrument are focused at a point 44 in thecenter of an image slit 98. An electron multiplier 100 is located so asto receive ions passing through the slit in retarding plate 48 and theslit in a grounded plate 102 located on the side of plate 44 oppositethe image slit. Tap 104 is connected to the negative side of a highvoltage supply to accelerate ions passing through the slit and plate 102to a very high velocity thereby maximizing the efficiency at which theelectron multiplier operates. An electrometer amplifier 106 is connectedto the output of multiplier 100 for amplifying the signal detected atthe collector for subsequent transmission to suitable recordingapparatus.

As can be seen from FIG. 1, the entrant and emergent ion beam intersectat 108. However, it has been found that the probability of collisionbetween the entrant and exit beam is low and does not constitute aproblem in the operation of the instrument. To further reduce theprobability of such collisions, an alternate embodiment of the exitportion of the analyzer is shown in FIG. 5. In that figure are shown thepoles of magnet 30 and a pair of deflection plates 110 located on eitherside of the emergent beam. Connection of a potential according to thepolarity shown in FIG. 5 produces a deflection of a resolved emergentbeam of positively charged particles as shown and a deflection of theimage point to position 1 12 as shown in FIG. 5. The undeflectedposition 114 of the image point (no voltage on deflection plates) isalso shown in phantom, together with its relation to the cross section116 of the entrant ion beam.

What is claimed is:

l. A mass analyzer comprising a curved magnetic analyzing sector fordeflecting charged particles directed into said analyzing sector througha total angle of deflection between and 360, said analyzing sectorhaving an entrance boundary and an exit boundary, the angulardisplacement of said exit boundary from said entrance boundary beingless than the total angle of deflection of the charged particles;

an object aperture for defining a point source of charged particles,said object aperture being spaced a first predetermined distance fromsaid entrance boundary and being positioned with respect to saidentrance boundary so as to direct particles towards said analyzingsector at a first predetermined angle to a normal to said entranceboundary; an image aperture spaced a second predetermined distance fromsaid exit boundary and positioned with respect to said exit boundary soas to receive emerging particles from the sector at a secondpredetermined angle to a normal to said exit boundy;

at least one of the said first and second predetermined angles being anacute angle;

said image aperture, object aperture and analyzing sector defining acharged particle trajectory having a first point of focus within saidanalyzing sector for particles of a predetermined mass and a secondpoint of focus for such particles at said image aperture; and

means located within said analyzing sector, associated with the firstpoint of focus, for selectively retarding the passage of chargedparticles having a mass different from such predetermined masstherebeyond;

said first and second predetermined distances, said first and secondpredetermined angles, the angular displacement of said exit boundaryfrom said entrance boundary, and the position of said selectivelyretarding means being chosen so that both median and Z focusing of thecharged particle trajectory occurs at said second point of focus.

2. A mass analyzer according to Claim 1 further comprising a source ofcharged particles provided with charged particle accelerating andfocusing means for directing the charged particles emanating from saidsource toward a point offocus at said object aperture.

3. A mass analyzer according to claim 2 further including a collector ofcharged particles located after said image aperture and a retardingmeans located between said image aperture and said collector forpreventing particles having less than a predetermined momentum frompassing therebeyond.

4. An analyzer according to claim 3 wherein said selectively retardingmeans located within the analyzing sector defines a pair of symmetricalanalyzing subsectors subtending equal angles of deflection between theentrance and exit boundaries.

5. A mass analyzer according to claim 4 wherein the magnetic analyzingsector is arranged so as to produce a deflection of 270 for chargedparticles introduced therein.

6. A mass analyzer according to claim 5 wherein the object aperture isspaced from the entrance boundary a distance such that the ratio of saiddistance to the radius of curvature in the analyzer of particles of thepredetermined mass is 1.998.

7. A mass analyzer according to claim 6 wherein the image aperture isspaced a distance from the exit boundary such that the ratio of saiddistance to the radius of curvature in the analyzer of particles of thepredetermined mass is 1.998 whereby the analyzer has stigmatic focusingfor particles of the predetermined mass.

8. A mass analyzer according to claim 7 wherein the first predeterminedangle to a normal to the entrance boundary is 26 35'.

9. A mass analyzer according to claim 8 wherein the second predeterminedangle to a normal to the exit boundary is 2635.

10. A mass analyzer according to claim 9 wherein the I collector is anelectron multiplier.

11. A mass analyzer according to claim 10 including electrostatic meansfor deflecting charged particles emerging from the exit boundary of theanalyzer to prevent the intersection of said particles with particlespassing from the source to the entrance boundary of the analyzer.

12. A mass analyzer according to claim 4 wherein the magnetic analyzingsector is arranged so as to produce a deflection of 240 for chargedparticles introduced therein.

13. A mass analyzer according to claim 4 wherein the magnetic analyzingsector is arranged so as to produce a deflection of 300 for chargedparticles introduced therein.

14. A tracer gas leak detector operating on a tandem mass analyzerprinciple comprising:

a magnet having a pair of spaced apart pole pieces arranged to producean angle of deflection of 270 between entrance and exit pole faces forcharged particles introduced into the space between the pole pieces, thepole pieces defining a charged particle analyzing sector therebetween;

a source for providing charged particles to be analyzed positioned withrespect to the entrance pole faces so as to direct particles between thepole pieces at an angle of 26 35 to a normal to the entrance pole faces,the source including means for admitting a sample of particles includingtracer gas particles to be analyzed into the source, means for producingcharged particles from particles in the sample, and means foraccelerating the charged particles from the source to a point of chargedparticle focus between the source and the entrance pole faces;

an object aperture located at said point of charged particle focus, theaperture being spaced a distance from said pole faces such that theratio of said distance to the radius of curvature of the tracer gascharged particle trajectory is 1.998;

an image aperture spaced from the exit pole faces, the aperture beingspaced a distance from said pole faces such that the ratio of saiddistance to the radius of curvature of the tracer gas charged particletrajectory is 1.998;

a secondary emission collector for tracer gas charged particlespositioned with respect to the exit pole faces at an angle of 26 35' toa normal to the exit pole faces so as to receive particles emerging frombetween the pole pieces;

the source, collector and magnet defining a charged particle trajectoryhaving a first point of focus within the analyzing sector for tracer gascharged particles at an angle of deflection of 135 and a second point offocus for tracer gas charged particles at the image aperture; and

an intermediate aperture located within the analyzing sector coincidingwith the first point of focus for defining the boundary between a firstand second analyzing sector within the magnet.

15. A leak detector according to claim 14 wherein the tracer gas ishelium.

16. A leak detector according to claim 11 including an auxiliarycollector for detecting the presence of an alternate tracer gas at saidalternate tracer gass first point of focus.

17. A. leak detector according to claim 16 wherein

1. A mass analyzer comprising a curved magnetic analyzing sector fordeflecting charged particles directed into said analyzing sector througha total angle of deflection between 180* and 360*, said analyzing sectorhaving an entrance boundary and an exit boundary, the angulardisplacement of said exit boundary from said entrance boundary beingless than the total angle of deflection of the charged particles; anobject aperture for defining a point source of charged particles, saidobject aperture being spaced a first predetermined distance from saidentrance boundary and being positioned with respect to said entranceboundary so as to direct particles towards said analyzing sector at afirst predetermined angle to a normal to said entrance boundary; animage aperture spaced a second predetermined distance from said exitboundary and positioned with respect to said exit boundary so as toreceive emerging particles from the sector at a second predeterminedangle to a normal to said exit boundary; at least one of the said firstand second predetermined angles being an acute angle; said imageaperture, object aperture and analyzing sector defining a chargedparticle trajectory having a first point of focus within said analyzingsector for particles of a predetermined mass and a second point of focusfor such particles at said image aperture; and means located within saidanalyzinG sector, associated with the first point of focus, forselectively retarding the passage of charged particles having a massdifferent from such predetermined mass therebeyond; said first andsecond predetermined distances, said first and second predeterminedangles, the angular displacement of said exit boundary from saidentrance boundary, and the position of said selectively retarding meansbeing chosen so that both median and Z focusing of the charged particletrajectory occurs at said second point of focus.
 2. A mass analyzeraccording to Claim 1 further comprising a source of charged particlesprovided with charged particle accelerating and focusing means fordirecting the charged particles emanating from said source toward apoint of focus at said object aperture.
 3. A mass analyzer according toclaim 2 further including a collector of charged particles located aftersaid image aperture and a retarding means located between said imageaperture and said collector for preventing particles having less than apredetermined momentum from passing therebeyond.
 4. An analyzeraccording to claim 3 wherein said selectively retarding means locatedwithin the analyzing sector defines a pair of symmetrical analyzingsubsectors subtending equal angles of deflection between the entranceand exit boundaries.
 5. A mass analyzer according to claim 4 wherein themagnetic analyzing sector is arranged so as to produce a deflection of270* for charged particles introduced therein.
 6. A mass analyzeraccording to claim 5 wherein the object aperture is spaced from theentrance boundary a distance such that the ratio of said distance to theradius of curvature in the analyzer of particles of the predeterminedmass is 1.998.
 7. A mass analyzer according to claim 6 wherein the imageaperture is spaced a distance from the exit boundary such that the ratioof said distance to the radius of curvature in the analyzer of particlesof the predetermined mass is 1.998 whereby the analyzer has stigmaticfocusing for particles of the predetermined mass.
 8. A mass analyzeraccording to claim 7 wherein the first predetermined angle to a normalto the entrance boundary is 26* 35''.
 9. A mass analyzer according toclaim 8 wherein the second predetermined angle to a normal to the exitboundary is 26*35''.
 10. A mass analyzer according to claim 9 whereinthe collector is an electron multiplier.
 11. A mass analyzer accordingto claim 10 including electrostatic means for deflecting chargedparticles emerging from the exit boundary of the analyzer to prevent theintersection of said particles with particles passing from the source tothe entrance boundary of the analyzer.
 12. A mass analyzer according toclaim 4 wherein the magnetic analyzing sector is arranged so as toproduce a deflection of 240* for charged particles introduced therein.13. A mass analyzer according to claim 4 wherein the magnetic analyzingsector is arranged so as to produce a deflection of 300* for chargedparticles introduced therein.
 14. A tracer gas leak detector operatingon a tandem mass analyzer principle comprising: a magnet having a pairof spaced apart pole pieces arranged to produce an angle of deflectionof 270* between entrance and exit pole faces for charged particlesintroduced into the space between the pole pieces, the pole piecesdefining a charged particle analyzing sector therebetween; a source forproviding charged particles to be analyzed positioned with respect tothe entrance pole faces so as to direct particles between the polepieces at an angle of 26* 35'' to a normal to the entrance pole faces,the source including means for admitting a sample of particles includingtracer gas particles to be analyzed into the source, means for producingcharged particles from particles in the sample, and means foraccelerating the charged particles from the source to a point of cHargedparticle focus between the source and the entrance pole faces; an objectaperture located at said point of charged particle focus, the aperturebeing spaced a distance from said pole faces such that the ratio of saiddistance to the radius of curvature of the tracer gas charged particletrajectory is 1.998; an image aperture spaced from the exit pole faces,the aperture being spaced a distance from said pole faces such that theratio of said distance to the radius of curvature of the tracer gascharged particle trajectory is 1.998; a secondary emission collector fortracer gas charged particles positioned with respect to the exit polefaces at an angle of 26* 35'' to a normal to the exit pole faces so asto receive particles emerging from between the pole pieces; the source,collector and magnet defining a charged particle trajectory having afirst point of focus within the analyzing sector for tracer gas chargedparticles at an angle of deflection of 135* and a second point of focusfor tracer gas charged particles at the image aperture; and anintermediate aperture located within the analyzing sector coincidingwith the first point of focus for defining the boundary between a firstand second analyzing sector within the magnet.
 15. A leak detectoraccording to claim 14 wherein the tracer gas is helium.
 16. A leakdetector according to claim 11 including an auxiliary collector fordetecting the presence of an alternate tracer gas at said alternatetracer gas''s first point of focus.
 17. A leak detector according toclaim 16 wherein alternate tracer gas is hydrogen.